1. Field of the Invention
The present invention relates to a toner for electrostatic image development which is employed in electrophotographic copying machines, printers, and facsimiles, and a method of producing the same. The present invention also relates to a toner for electrostatic image development which can also be preferably employed in the development of a toner-jet printer.
2. Description of the Related Art
In electrophotographic copying machines, printers, and facsimiles, the following needs to the toner have recently been enhanced for cost reduction and size reduction of machines as well as power saving and resource saving, including a further improvement in quality of the printed image. The needs include improvement in definition and gradient of the printed image, reduction in thickness of the toner layer, reduction in amount of wasted toner, reduction in particle diameter and spheroidizing of the toner for reducing the amount of the toner consumed per page, decrease in fixing temperature for reduction in power consumed, oilless fixation for simplification of machines, improvement in hue, transparency and gloss in full-color image, reduction in VOC (volatile organic compound) during the fixation, which is likely to exert an adverse influence on human health, and the like.
Reduction in particle diameter of the powdered toner prepared by a pulverization method, which has been employed for a long time, can be basically carried out. However, with the reduction in particle size, there arise these problems that (1) it becomes difficult to control charge because of an increase in the amount of colorants and waxes exposed on the surface of toner particles, (2) the fluidity of the powder is lowered by the irregular shape of the toner particles and (3) the energy cost required for the production increases, thus making it difficult to sufficiently satisfy the needs described above.
Therefore, a development of a spherical toner having a small particle diameter has been made intensively by the polymerization method or emulsification/dispersion method. Although various methods have been known as the method of producing a toner using the polymerization method, there has widely been employed the suspension polymerization method, which comprises uniformly dissolving and dispersing a monomer, a polymerization initiator, a colorant and a charge control agent, adding the mixture in an aqueous medium containing a dispersion stabilizer while stirring to form oil droplets and heating, thereby to cause the polymerization reaction to obtain toner particles. Although the reduction in particle diameter and spheroidizing can be satisfactorily conducted by the polymerization method, a principal component of the binder resin is limited to a radically-polymerizable vinyl polymer and toner particles made of a polyester resin or epoxy resin suited for use as a color toner cannot be produced by the polymerization method. It is difficult to reduce VOC (volatile organic compound made of an unreacted monomer) by the polymerization method and its improvement is required.
As is disclosed in Japanese Patent Application, First Publication No. Hei 5-66600 and Japanese Patent Application, First Publication No. Hei 8-211655, the method of producing the toner using the emulsification/dispersion method is a method of mixing a mixture of a binder resin and a colorant with an aqueous medium and emulsifying them to obtain toner particles and has these advantages that (1) possible binder resins can be widely selected, (2) reduction in VOC is easy to realize and (3) the concentration of the colorant is easy to change optionally within a range of low to high value, as compared with the polymerization method, in addition to the advantage that is easy to cope with the reduction in particle diameter and spheroidizing of the toner similar to the polymerization method.
It has generally been known that a polyester resin is more preferable than a styrene-acrylic resin as a binder resin for a toner, which can reduce the fixing temperature and forms a smooth image surface by sharp melting during the fixation, and a polyester resin having an excellent pliability is used in the color toner particularly preferably. As described above, since the polyester resin can not be employed as the principal component of the binder resin in the polymerization method, a spherical or generally spherical toner having a small particle diameter containing a polyester resin as the binder resin obtained by the emulsification/dispersion method has attracted special interest recently.
However, a polyester resin toner obtained by the emulsification/dispersion method which has hitherto been employed contains a straight-chain resin having a comparatively low molecular weight as the binder resin. Therefore, it is essential to coat a fixing heat roller with an anti-offset solution such as silicone oil and the silicone oil transfers to a printing paper or an OHP paper. So in addition to the problem of maintenance, are problems such as poor writing on printed sheet and greasiness due to oil. There was also a problem that the peel strength is not sufficient necessarily because it varies depending on the purposes.